For a number of years the coating industry has been engaged in substantial developmental programs in the quest for procedures which would allow the coating of substrates at high production rates with the coating being cured to a tack-free condition at a speed commensurate with the contiguous processing steps. The industry has desired to eliminate the volatile solvents required in many of the well known coating processes because of potential hazards or because of the cost of equipment to handle the evolved solvent vapors. In addition, the industry has been seeking coating formulations which would produce coatings which were durable and which would permit substantial additional processing of the workpiece, such as metal forming operations where the substrate is metal strip and container bodies, blanks and closures.
Epoxy coating formulations have long been recognized as affording desirable properties in the finished coating, especially the toughness to withstand further processing. However, the problem has remained to develop a low cost epoxy coating formulation which would combine the desired rheological properties for the coating application with both reasonable pot life and rapid curing in the production line.
In Watt United States Letters Pat. No. 3,794,576 granted Feb. 26, 1974, there are described desirable epoxy formulations which combine the desired rheological properties with suitable pot life and rapid curing. The formulations contain a Lewis acid precursor catalyst which is decomposed upon irradiation by ultraviolet light to produce rapid curing of the coating to a tack free condition. However, to achieve the desired rapid curing, the epoxide formulations therein contain at least about 15 percent by weight of an epoxidic ester having two epoxycycloalkyl groups; such esters materially increase the cost of the formulation as compared with the more conventional epoxide prepolymer materials.
Since the disclosure of Watt, a number of patents and publications have appeared proposing various catalyst precursors for the epoxy formulations which could replace the diazonium catalysts specifically described in the Watt Patent. Among these are the onium catalysts disclosed in Barton United States Letters Pat. No. 4,090,936 granted May 23, 1978; Crivello U.S. Letters Pat. Nos. 4,069,055 granted Jan. 17, 1978 and No. 4,058,401 granted Nov. 15, 1977. However, the search has continued for low cost formulations which would cure rapidly, i.e., formulations which would not require even small amounts of special components such as the epoxidic esters of the Watt Patent.
Although it has long been known that heat will accelerate the reaction rate of polymerization following irradiation to effect complete curing, generally in accordance with the Arrhenius equation, and has long been suggested to augment the irradiation treatment (see, for example, the aforementioned Watt Patent at Column 6, lines 1-16), such heating has also been recognized to introduce other problems in the form of production requirements and possible adverse effects upon the polymer properties since low molecular weight polymers may result. Some researchers have evaluated various factors affecting cure rate in such irradiated epoxide formulations and have proposed substantially elevated temperatures to increase the cure rate while avoiding volatilization of the monomer (See, Crivello et al, "Triaryl Sulfonium Salts: A New Class of Photoinitiators for Cationic Polymerization", JOURNAL OF RADIATION CURING, Volume 5, pages 2, 10-11, January 1978).
Although elevated temperatures of 100.degree. C. and above would appear to be advantageous in terms of acceleration of reaction rate, such temperatures have been found to substantially affect the quality of the polymeric coating although no monomer or volatile component may be driven off during the process. Moreover, elevation of the coating to such temperatures and maintenance thereat for any appreciable length of time presents substantial production problems when high speed processing is involved.
Accordingly, it is an object of the present invention to provide a process for coating substrates with relatively low cost epoxy prepolymer formulations which are activated by irradiation and which will produce a tack free surface condition rapidly for use on high speed production equipment.
It is also an object to provide such a process which permits the utilization of relatively low cost epoxy prepolymers and a wide range of ultraviolet sensitive catalyst precursors.
Another object is to provide such a process which may be adapted to a wide variety of high speed coating lines and which does not require extensive or expensive equipment.